In military terminology, a sentry (previously known as a sentinel) is a member of personnel placed in some post to watch the approach of the enemy, to prevent surprises, and to stop other personell passing without orders or strangers passing. A sentry placed in an extremely hazardous location is known as a sentinel perdu.
An 18th century military manual eloquently illustrates the precautions and seriousness of the duty of a sentry, describing the duties of a sentry thus:
All sentines [sentries] are to be vigilant on their posts; neither are they to sing, smoke tobacco, nor suffer any noise to be made nrear them. They ate to have a watchful eye over the things committed to their charge. They are not to suffer any light to remain, or any fire to be made, near their posts in the night-time; neither is any sentry to be relieved or removed from his post but by the corporal of the guard. They are not to suffer any one to touch or handle their arms, or in the night-time to come within ten yards (nine meters) of their post.
No person is to strike or abuse a sentry on his post; but when he has committeda crime, he is to be relieved, and then punished according to the rules and articles of war.
A sentinel, on his post in the night, is to know nobody but by the counter-sign: when he challenges, and is unaswered, Relief, he calls out, Stand relief! Advance, corporal! Upon which the corporal halts his men, and advances alone within a yard (one meter) of the sentry's firelock (first ordering his party to rest, on which the sentry does the same), and gives him the counter-sign, taking care that no one hear it. Research Sentry
Abbreviations
Warfare
Research Sentry